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1.
An especially fine woman's butt.
So ladies (yeah), ladies (yeah)
If you wanna role in my Mercedes (yeah)
Then turn around
Stick it out
Even white boys got to shout
Baby got back

All credit humbly given to Sir-Mix-a-Lot
by baby got it goin' on February 13, 2004
 
2.
A reference to the buttocks, usually referred to woman who have a bigger than normal ass.
"Daaaammmnnnnn, baby got back!"
by Alex A. February 18, 2003
 
3.
Is a term used by (usually) pot smokers though it can be used when a group of people are smoking (sharing) anything. The person who "calls" "backs" gets to take second hit. Kind of like calling "shotgun" for passenger seat of a car...
*Todd prepares to spark the blunt..

Rodd: I got backs!

Dodd: Fuck, I got trips

Dick: shit that means I'm last. Technically this is better for me because the smoke from the smoked weed passes through the blunt and deposits THC resin on the bud that it passes through to get the mouth end of the blunt. This means my first hit will yield more THC for me. Yet, it also means that I will have a smaller number of total hits on the blunt. This means I must chief on the blunt like a motherfucker.
by ysf June 17, 2009
 
4.
A (navy) call made when you have to get past someone in the passageway, or to announce your presence in the room.
As Roof enters the mess he would call "backs" which would result in a response of "backs" by every member in the mess. A non-response generally means that the paticular members sexuality is questioned.

"Backs, big unit coming through" as Parko or Reid pass you on the way to 2 Mess.
by newcastlecoons August 31, 2008
 
5.
A back is a non-alcoholic drink served alongside a shot. Backs are also sometimes called chasers. Sometimes a small beer served alongside a shot is considered a back.
Gimme a shot of whiskey with a coke back.
by Learn to Swim August 12, 2008
 
6.
it is reffered to somes ones bum,
either boy or girl
*girl with big bum*
"look that girls got backs"
by bbygyal123456 January 28, 2009
 
7.
a slang word for the US Dollar.
Derived from the word greenbacks, for the USA's green paper bills.
by Alicia Notforbush August 02, 2005